Some new designs of mobile communication devices—such as smart phones, tablet computers, and laptop computers—contain two or more Subscriber Identity Module (“SIM”) cards that provide users with access to multiple separate mobile telephony networks. Examples of mobile telephony networks include Third Generation (3G), Fourth Generation (4G), Long Term Evolution (LTE), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), Wideband CDMA (WCDMA), Global System for Mobile Communications (GSM), and Universal Mobile Telecommunications Systems (UMTS). Example multi-SIM mobile communication devices include mobile phones, laptop computers, smart phones, and other mobile communication devices that are configured to connect to multiple mobile telephony networks. A mobile communication device that includes a plurality of SIMs and connects to two or more separate mobile telephony networks using two or more separate radio-frequency (“RF”) transceivers is termed a “multi-SIM-multi-active” or “MSMA” communication device. An example MSMA communication device is a “dual-SIM-dual-active” or “DSDA” communication device, which includes two SIM cards/subscriptions associated with two mobile telephony networks.
Because a multi-SIM-multi-active communication device has a plurality of separate RF communication circuits or “RF resources,” each subscription on the multi-SIM-multi-active communication device may use an associated RF resource to communicate with the subscription's mobile network at any time. However, in some circumstances, the multi-SIM-multi-active communication device may operate in a transmit-sharing (“Tx-sharing”) mode in which two (or more) subscriptions share a single transmit (“Tx”) resource (e.g., a transmitter or transceiver). While operating in a Tx-sharing mode, a first subscription (e.g., an LTE subscription) and a second subscription (e.g., a GSM subscription) cannot simultaneously utilize the shared Tx resource. As a result, the first subscription and/or the second subscription typically experience degraded transmit performance in situations in which both subscriptions need to use the Tx resource at the same time, such as when both subscriptions are engaged in a data or voice call.